I order online very often, and I must say for the USPS, approximate is the right term to describe their tracking system. I've checked USPS tracking many times. Sometimes it said the package was already delivered, and it still didn't come for a week. Other times, it said delivery was next week, and it came that day.

In comparison, UPS tracking is right on time, almost to the minute.

So for the USPS, tracking is essentially useless, they need some Perl programmers and some scanners. :-)